Aggressive and sexual social stimuli do not phase shift the circadian temperature rhythm in rats

Chronic Social Defeat Stress Shifts Peripheral Circadian Clocks in Male Mice in a Tissue-Specific and Time-of-Day Dependent Fashion

Uncontrollable stress is linked to the development of many diseases, some of which are associated with disrupted daily rhythms in physiology and behavior. While available data indicate that the master circadian pacemaker in the suprachiasmatic nucleus (SCN) is unaffected by stress, accumulating evidence suggest that circadian oscillators in peripheral tissues and organs can be shifted by a variety of stressors and stress hormones. In the present study, we examined effects of acute and chronic social defeat stress in mice and addressed the question of whether effects of uncontrollable stress on peripheral clocks are tissue specific and depend on time of day of stress exposure. We used mice that carry a luciferase reporter gene fused to the circadian clock gene Period2 (PER2::LUC) to examine daily rhythms of PER2 expression in various peripheral tissues. Mice were exposed to social defeat stress in the early (ZT13-14) or late (ZT21-22) dark phase, either once (acute stress) or repeatedly on 10 consecutive days (chronic stress). One hour after the last stressor, tissue samples from liver, lung, kidney, and white adipose tissue (WAT) were collected. Social defeat stress caused a phase delay of several hours in the rhythm of PER2 expression in lung and kidney, but this delay was stronger after chronic than after acute stress. Moreover, shifts only occurred after stress in the late dark phase, not in the early dark phase. PER2 rhythms in liver and WAT were not significantly shifted by social defeat, suggesting a different response of various peripheral clocks to stress. This study indicates that uncontrollable social defeat stress is capable of shifting peripheral clocks in a time of day dependent and tissue specific manner. These shifts in peripheral clocks were smaller or absent after a single stress exposure and may therefore be the consequence of a cumulative chronic stress effect.

The impact of stress and stress hormones on endogenous clocks and circadian rhythms

In mammals, daily rhythms in physiology and behavior are under control of a circadian pacemaker situated in the suprachiasmatic nucleus (SCN). This master clock receives photic input from the retina and coordinates peripheral oscillators present in other tissues, maintaining all rhythms in the body synchronized to the environmental light-dark cycle. In line with its function as a master clock, the SCN appears to be well protected against unpredictable stressful stimuli. However, available data indicate that stress and stress hormones at certain times of day are capable of shifting peripheral oscillators in, e.g., liver, kidney and heart, which are normally under control of the SCN. Such shifts of peripheral oscillators may represent a temporary change in circadian organization that facilitates adaptation to repeated stress. Alternatively, these shifts of internal rhythms may represent an imbalance between precisely orchestrated physiological and behavioral processes that may have severe consequences for health and well-being.

Social stress and glucocorticoids alter PERIOD2 rhythmicity in the liver, but not in the suprachiasmatic nucleus

Circadian (~24 h) rhythms in behavior and physiological functions are under control of an endogenous circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN directly drives some of these rhythms or serves as a coordinator of peripheral oscillators residing in other tissues and organs. Disruption of the circadian organization may contribute to disease, including stress-related disorders. Previous research indicates that the master clock in the SCN is resistant to stress, although it is unclear whether stress affects rhythmicity in other tissues, possibly mediated by glucocorticoids, released in stressful situations. In the present study, we examined the effect of uncontrollable social defeat stress and glucocorticoid hormones on the central and peripheral clocks, respectively in the SCN and liver. Transgenic PERIOD2::LUCIFERASE knock-in mice were used to assess the rhythm of the clock protein PERIOD2 (PER2) in SCN slices and liver tissue collected after 10 consecutive days of social defeat stress. The rhythmicity of PER2 expression in the SCN was not affected by stress exposure, whereas in the liver the expression showed a delayed phase in defeated compared to non-defeated control mice. In a second experiment, brain slices and liver samples were collected from transgenic mice and exposed to different doses of corticosterone. Corticosterone did not affect PER2 rhythm of the SCN samples, but caused a phase shift in PER2 expression in liver samples. This study confirms earlier findings that the SCN is resistant to stress and shows that clocks in the liver are affected by social stress, which might be due to the direct influence of glucocorticoids released from the adrenal gland.

Chronic social defeat stress suppresses locomotor activity but does not affect the free-running circadian period of the activity rhythm in mice

In mammals, daily rhythms in behavior and physiology are under control of an endogenous clock or pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN assures an optimal temporal organization of internal physiological process and also synchronizes rhythms in physiology and behavior to the cyclic environment. The SCN receives direct light input from the retina, which is capable of resetting the master clock and thereby synchronizes internally driven rhythms to the external light-dark cycle. In keeping with its function as a clock and pacemaker, the SCN appears to be well buffered against influences by other stimuli and conditions that contain no relevant timing information, such as acute stressors. On the other hand, it has been suggested that chronic forms of stress may have gradually accumulating effects that can disturb normal clock function and thereby contribute to stress-related disorders. Therefore, in the present study we investigated whether chronic intermittent social stress affects the endogenous period and phase of the free-running activity rhythm in mice. Adult male mice were maintained in constant dim red light conditions and exposed to a daily 20 min social defeat stress session for 10 consecutive days, either during the first half of their activity phase or the first half of their resting phase. The overall amount of running wheel activity was strongly suppressed during the 10 days of social defeat, to about 50% of the activity in non-defeated control mice. Activity levels gradually normalized during post-defeat recovery days. Despite the strong suppression of activity in defeated animals, the endogenous free-running circadian period of the activity rhythm and the phase of activity onset were not affected. These findings are thus in agreement with earlier studies suggesting that the circadian pacemaker in the SCN that is driving the rhythmicity in activity is well-protected against stress. Even severe social defeat stress for 10 consecutive days, which has a major effect on the levels of activity, does not affect the pace of the endogenous clock.

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Chronic intermittent social defeat stress strongly reduces locomotor activity in mice.

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Chronic intermittent social defeat stress does not affect the circadian period and phase of the free-running activity rhythm under constant conditions.

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Social defeat stress does not appear to affect the endogenous clock in the SCN driving the rhythmicity in activity.

Repeated Psychosocial Stress at Night Affects the Circadian Activity Rhythm of Male Mice

We have recently shown that molecular rhythms in the murine suprachiasmatic nucleus (SCN) are affected by repeated social defeat (SD) during the dark/active phase (social defeat dark [SDD]), while repeated SD during the light/inactive phase (social defeat light [SDL]) had no influence on PERIOD2::LUCIFERASE explant rhythms in the SCN. Here we assessed the effects of the same stress paradigm by in vivo biotelemetry on 2 output rhythms of the circadian clock (i.e., activity and core body temperature) in wild-type (WT) and clock-deficient Period (Per)1/2 double -mutant mice during and following repeated SDL and SDD. In general, stress had more pronounced effects on activity compared to body temperature rhythms. Throughout the SD procedure, activity and body temperature were markedly increased during the 2 h of stressor exposure at zeitgeber time (ZT) 1 to ZT3 (SDL mice) and ZT13 to ZT15 (SDD mice), which was compensated by decreased activity during the remaining dark phase (SDL and SDD mice) and light phase (SDL mice) in both genotypes. Considerable differences in the activity between SDL and SDD mice were seen in the poststress period. SDD mice exhibited a reduced first activity bout at ZT13, delayed activity onset, and, consequently, a more narrow activity bandwidth compared with single-housed control (SHC) and SDL mice. Given that this effect was absent in Per1/2 mutant SDD mice and persisted under constant darkness conditions in SDD WT mice, it suggests an involvement of the endogenous clock. Taken together, the present findings demonstrate that SDD has long-lasting consequences for the functional output of the biological clock that, at least in part, appear to depend on the clock genes Per1 and Per2.

Integration of biological clocks and rhythms

Animals, plants, and microorganisms exhibit numerous biological rhythms that are generated by numerous biological clocks. This article summarizes experimental data pertinent to the often-ignored issue of integration of multiple rhythms. Five contexts of integration are discussed: (i) integration of circadian rhythms of multiple processes within an individual organism, (ii) integration of biological rhythms operating in different time scales (such as tidal, daily, and seasonal), (iii) integration of rhythms across multiple species, (iv) integration of rhythms of different members of a species, and (v) integration of rhythmicity and physiological homeostasis. Understanding of these multiple rhythmic interactions is an important first step in the eventual thorough understanding of how organisms arrange their vital functions temporally within and without their bodies.

Timekeeping through social contacts: social synchronization of circadian locomotor activity rhythm in the carpenter ant Camponotus paria

In ant colonies a large proportion of individuals remain inside nests for most of their lives and come out only when necessary. It is not clear how, in a nest of several thousand individuals, information about local time is communicated among members of the colony. Central to this seem to be circadian clocks, which have an intrinsic ability to keep track of local time by entraining to environmental light-dark, temperature, and social cycles. Here, the authors report the results of their study aimed at understanding the role of cyclic social interactions in circadian timekeeping of a day-active species of carpenter ant Camponotus paria. The authors found that daily social interactions with visitors (worker ants) was able to synchronize the circadian locomotor activity rhythm of host worker ants and queens, in one-on-one (pair-wise) and multi-individual (group-wise) interactions. Interestingly, the outcome of cyclic social interactions was context specific; when visitor workers socially interacted with host workers one-on-one, host workers considered the time of interaction as subjective day, but when visitor workers interacted with a group of workers and queens, the hosts considered the time of interaction as subjective night. These results can be taken to suggest that members of the ant species C. paria keep track of local time by socially interacting with workers (foragers) who shuttle in and out of the colony in search of food. (Author correspondence: vsharma@jncasr.ac.in ).

Social synchronization of circadian locomotor activity rhythm in the fruit fly Drosophila melanogaster

Circadian clocks regulate the physiology and behaviour of organisms across a wide range of taxa. To keep track of local time, these clocks use a variety of time cues such as light-dark, temperature, food availability and social interaction cycles. This study assessed the role of social cues in modulating circadian clocks of the fruit fly Drosophila melanogaster. Using pair-wise interactions, we first estimated the percentage contribution of each interacting partner on the cumulative rhythmic behaviour of the pairs. Subsequently, we studied the effects of multi-individual (group-wise) interactions on the rhythmic behaviour of the group by estimating phase synchrony between individuals of different strains (having different circadian periods) maintained in both homogeneous and heterogeneous groups. Although it is known that social interactions improve synchrony between interacting individuals, we asked whether such interactions are able to synchronize the circadian rhythms of highly phase-desynchronized flies. We found that, although interactions between fly strains possessing different circadian periods failed to produce synchrony, social interactions among phase-desynchronized flies did enhance the phase synchrony of the interacting individuals. Differently phased individuals living in social groups displayed significantly greater phase synchrony than those living solitarily. Social synchronization is olfaction mediated as group-wise interactions among phase-desynchronized flies possessing compromised olfactory ability (Or83b(0)) did not improve phase synchrony. These results suggest that social cues synchronize the circadian clocks of Drosophila provided that the interacting individuals have similar clock periods.

Social interaction with a rhythmic rat enhances the circadian pattern of the motor activity and temperature of LL-induced arrhythmic rats

Although light is the main factor that influences circadian rhythms, social interaction may also have a role on their regulation. Here, the influence of social interaction on rat circadian behavior was investigated, addressing the question of whether cohabitation would induce the appearance of a circadian rhythm in arrhythmic rats due to constant light. To this end, circadian rhythms of motor activity and body temperature of male and female LL-induced arrhythmic rats were studied before, during and after a 20-day period in which rats stayed in the same cage with a rat of the same sex but with stronger rhythm. Results showed that the manifestation of the circadian motor activity rhythm of LL-induced arrhythmic rats increased after cohabitation. In the case of the expression of the body temperature rhythm, there was a progressive daily increase in the power content of a daily 24 hour pattern throughout the cohabitation days, which remained when animals were again isolated. Thus, the presence of a rhythmic rat increases the strength of the circadian behavior of rats showing a weak circadian rhythm.

Social influences on mammalian circadian rhythms: animal and human studies

While light is considered the dominant stimulus for entraining (synchronizing) mammalian circadian rhythms to local environmental time, social stimuli are also widely cited as 'zeitgebers' (time-cues). This review critically assesses the evidence for social influences on mammalian circadian rhythms, and possible mechanisms of action. Social stimuli may affect circadian behavioural programmes by regulating the phase and period of circadian clocks (i.e. a zeitgeber action, either direct or by conditioning to photic zeitgebers), by influencing daily patterns of light exposure or modulating light input to the clock, or by associative learning processes that utilize circadian time as a discriminative or conditioned stimulus. There is good evidence that social stimuli can act as zeitgebers. In several species maternal signals are the primary zeitgeber in utero and prior to weaning. Adults of some species can also be phase shifted or entrained by single or periodic social interactions, but these effects are often weak, and appear to be mediated by social stimulation of arousal. There is no strong evidence yet for sensory-specific nonphotic inputs to the clock. The circadian phase-dependence of clock resetting to social stimuli or arousal (the 'nonphotic' phase response curve, PRC), where known, is distinct from that to light and similar in diurnal and nocturnal animals. There is some evidence that induction of arousal can modulate light input to the clock, but no studies yet of whether social stimuli can shift the clock by conditioning to photic cues, or be incorporated into the circadian programme by associative learning. In humans, social zeitgebers appear weak by comparison with light. In temporal isolation or under weak light-dark cycles, humans may ignore social cues and free-run independently, although cases of mutual synchrony among two or more group-housed individuals have been reported. Social cues may affect circadian timing by controlling sleep-wake states, but the phase of entrainment observed to fixed sleep-wake schedules in dim light is consistent with photic mediation (scheduled variations in behavioural state necessarily create daily light-dark cycles unless subjects are housed in constant dark or have no eyes). By contrast, discrete exercise sessions can induce phase shifts consistent with the nonphotic PRC observed in animal studies. The best evidence for social entrainment in humans is from a few totally blind subjects who synchronize to the 24 h day, or to near-24 h sleep-wake schedules under laboratory conditions. However, the critical entraining stimuli have not yet been identified, and there are no reported cases yet of social entrainment in bilaterally enucleated blind subjects. The role of social zeitgebers in mammalian behavioural ecology, their mechanisms of action, and their utility for manipulating circadian rhythms in humans, remains to be more fully elaborated.

Reward and Aversive Stimuli Produce Similar Nonphotic Phase Shifts

Circadian rhythms in rodents respond to arousing, nonphotic stimuli that contribute to daily patterns of entrainment. To examine whether the motivational significance of a stimulus is important for eliciting nonphotic circadian phase shirts in Syrian hamsters (Mesocricetus auratus), the authors compared responses to a highly rewarding stimulus (lateral hypothalamic brain stimulation reward [BSR]) and a highly aversive stimulus (footshock). Animals were housed on a 14:10-hr light-dark cycle until test day, when they were given a 1-hr BSR session (trained animals) or a 1-mA electric footshock at 1 of 8 circadian times, and were maintained in constant dark thereafter. Both BSR pulses and footshock produced nonphotic phase response curves. These results support the hypothesis that arousal resulting from the motivational significance of a stimulus is a major factor in nonphotic phase shifts.

Birds of a feather clock together – sometimes: social synchronization of circadian rhythms

Biological systems use internal circadian clocks to efficiently organize physiological and behavioral activity within the 24-hour time domain. In the absence of time cues, circadian periods vary slightly from 24 hours, but in nature, ambient light serves as the most salient synchronizer for these rhythms, fine-tuning them to exactly 24 hours each day. For some species, social cues can serve to synchronize circadian rhythms in the absence of other time cues or to amplify ambiguous light cues. This has been demonstrated to various degrees in fruit flies, degus, birds, fish, bats, beavers and humans; however, studies in rats and hamsters have shown that social cues are less salient time cues for these species. Social influences on circadian timing might function to tightly organize the social group, thereby decreasing the chances of predation and increasing the likelihood of mating.

Circadian clock resetting by arousal in Syrian hamsters: the role of stress and activity

Circadian rhythms in the Syrian hamster can be markedly phase shifted by 3 h of wheel running or arousal stimulation during their usual daily rest period ("subjective day"). Continuous wheel running is predictive but not necessary for phase shifts of this "nonphotic" type; hamsters aroused by gentle handling without running can also show maximal shifts. By contrast, physical restraint, a standard stress procedure and thus presumably arousing, is ineffective. To resolve this apparent paradox, phase-shifting effects of 3-h sessions of restraint or other stress procedures were assessed. In a preliminary study, phase shifts to arousal by gentle handling were significantly potentiated by the cortisol synthesis inhibitor metyrapone, suggesting that stress-related cortisol release may inhibit phase shifts to arousal. Next, it was confirmed that restraint in the subjective day does not induce phase shifts, but behavioral observations revealed that it also does not sustain arousal. Restraint combined with noxious compressed air blasts did sustain arousal and induced a significant cortisol response compared with arousal by gentle handling but did not induce shifts. Restraint combined with continuous horizontal rotation was also ineffective, as was EEG-validated arousal via confinement to a pedestal over water. However, 3 h of resident-intruder interactions (an intense psychosocial stress) or exposure to an open field (a mild stress) did induce large shifts that were positively correlated with indexes of forward locomotion. The results indicate that large phase shifts associated with arousal in the usual sleep period are neither induced nor prevented by stress per se, but are dependent on the expression of at least low levels of locomotor activity. Sustained arousal alone is not sufficient.

Effects of social stimuli on sleep in mice: non-rapid-eye-movement (NREM) sleep is promoted by aggressive interaction but not by sexual interaction

Sleep is generally considered to be a process of recovery from prior wakefulness. In addition to being affected by the duration of the waking period, sleep architecture and sleep EEG also depend on the quality of wakefulness. In the present experiment, we examined how sleep is affected by different social stimuli (social conflict and sexual interaction). Male C57BL/6J mice were placed in the cage of an aggressive dominant male or an estrous female for 1 h in the middle of the light phase. The conflict with an aggressive male had a pronounced NREM sleep-promoting effect. EEG slow wave activity, a measure of NREM sleep intensity, was increased for about 6 h and NREM sleep time was significantly increased for 12 h. REM sleep was strongly suppressed during the remainder of the light phase after the conflict, followed by a rebound later in the recovery phase. The sexual interaction, in contrast, had only mild effects. Both NREM sleep and REM sleep were somewhat suppressed shortly after the interaction. In a separate group of mice, blood samples were taken to measure prolactin and corticosterone. The results suggest that the temporary suppression of REM sleep following the social stimuli may be partly due to elevated corticosterone. The different effects of the social stimuli on NREM sleep are not easily explained by differences in the hormone responses. In conclusion, although both social conflict and sexual interaction induce a strong physiological activation, only social conflict has a strong stimulatory effect on NREM sleep mechanisms.

Circadian clock resetting by sleep deprivation without exercise in the Syrian hamster

Circadian rhythms in several species can be phase-shifted by procedures that stimulate locomotor activity ("exercise") during the usual sleep period. The role of arousal or sleep loss, independent of activity, in this effect has not been adequately resolved. We show here, using the sleep deprivation procedure of gentle handling, that comparably large phase shifts (up to 240 min advances) of the rest-activity cycle can be induced in Syrian hamsters by 3 hr of behavioral arousal, with minimal locomotion, beginning 6 hr before the usual active period. Horizontal distance traveled during the deprivation procedure averaged approximately 0.08 km, compared to 2. 5 km typical in exercise studies. Hamsters requiring fewer interventions exhibited larger shifts, suggesting that the level or continuity of spontaneous arousal determines shift size. The circadian rhythm of light-induced c-fos expression in the suprachiasmatic nucleus (SCN) was used as a phase marker to further demonstrate that the clock is reset within 1 hr after a 3 hr deprivation. Sleep deprivation mimicked the effects of exercise on basal c-fos expression in two components of the circadian system, suppressing basal Fos immunoreactivity in the SCN, and increasing Fos in the intergeniculate leaflet. Sleep deprivation without exercise in hamsters can rapidly reset the circadian clock and alter gene expression within the circadian system.

Local effects of the serotonin agonist quipazine on the suprachiasmatic nucleus of rats

In mammals, circadian rhythms of locomotor activity and many other behavioral and physiological functions are controlled by an endogenous pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). One of the SCN's afferents is a dense serotonergic input from the mesencephalic raphe complex. Previous work from this laboratory demonstrated that systemic administrations of the serotonin agonist quipazine mimic the effects of light on the circadian system of rats, i.e. they induce photic-like phase shifts of the circadian activity rhythm as well as c-Fos expression in the SCN. In contrast, no such effect has been demonstrated so far in the isolated rat SCN slice preparation. In this study we demonstrate that local injections of quipazine (0.5 microg/kg) into the region of the SCN induce photic-like effects similar to those induced by systemic injections. These findings suggest a role for 5-HT in the transmission of photic information to the rat circadian system through a direct action at the level of the SCN.

Serotonin agonist quipazine induces photic-like phase shifts of the circadian activity rhythm and c-Fos expression in the rat suprachiasmatic nucleus

Nonphotic stimuli can reset and entrain circadian activity rhythms in hamsters and mice, and serotonin is thought to be involved in the phase-resetting effects of these stimuli. In the present study, the authors examined the effect of the serotonin agonist quipazine on circadian activity rhythms in three inbred strains of rats (ACI, BH, and LEW). Furthermore, they investigated the effect of quipazine on the expression of c-Fos in the mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN). Quipazine reduced the amount of running wheel activity for 3 h after treatment, however, no long-term changes in tau and in the activity level were observed. More important, quipazine induced significant phase advances of the activity rhythm and c-Fos production in the SCN at the end of the subjective night (Circadian Time [CT] 22), whereas neither phase shifts nor c-Fos induction were observed during the subjective day. Quipazine injections also resulted in moderate phase delays at the beginning of the subjective night (CT 14). A similar phase-response characteristic typically can be observed for photic stimuli. By contrast, nonphotic stimuli normally produce phase advances during the subjective day. The present results suggest species differences between the hamster and the rat with respect to the serotonergic action on circadian timekeeping and indicate that serotonergic pathways play a role in the transmission of photic information to the SCN of rats.